inlude of activeMailLib fails on some systems

On very restrictive systems, the current way including activeMailLib fails, due to an open_basedir error.

It took me a while to figure out what went wrong. My system does not produce any error output (switched off).
Sending a newsletter to the test account failed an resulted in a blank screen.

I changed the way activeMailLib is included to this:

  $path = drupal_get_path('module', 'simplenews');
  require_once("./$path/activeMailLib.php");

Patch appended.